1. Field of the Invention
The present invention relates to an optical phase-change recording medium which records and erases information by changing a phase of a recording layer between an amorphous state and a crystalline state.
2. Description of the Related Art
A DVD-RAM, which permits recording, reproduction, and erasure of information, has been attracting wide attention as it has advantages over a DVD-ROM, which is only capable of reproduction, and a DVD-R, which is only capable of recording and reproduction.
In regard to an optical-disk drive, a scheme that obtains a tracking-error signal through phase-difference tracking is known in the art. Reference may be made to Japanese Patent Publication No. 56-30610 and Japanese Patent Publication No. 2-56734, the contents of which are hereby incorporated by reference. In this scheme, a laser-beam spot is scanned over a recording track, which is comprised of a series of reflection pits each formed as a lump or a recess. An optical phase of reflected light is then modulated so as to detect a change in an opticalintensity distribution observed on a receiver device. Based on this change, a displacement of the series of pits from an expected center thereof is detected. A position of the laser-beam spot is subjected to servo control to minimize the displacement, thereby effecting appropriate tacking.
No optical phase-change recording medium; however, is known to date which has a memory capacity comparable to that of the DVD-ROM and can be driven by a DVD-ROM drive.
One of the reasons why such medium does not exist is that optical phase-change recording media usually exhibit a low reflectivity and allow only a small degree of amplitude modulation to be obtained, as physical and signal characteristics are concerned. Such characteristics result in an insufficient signal magnitude. Because of this reason, it is difficult to drive a DVD-RAM in the same manner as driving DVD-ROM in a DVD-ROM drive. Particularly, a phase-difference tracking signal, which is necessary for tracking a series of pits, cannot be obtained in a sufficient magnitude, giving rise to a problem in that the tracking control becomes unstable.
Accordingly, there is a need for an optical phase-change medium which permits appropriate phase-difference tracking.
Accordingly, it is a general object of the present invention to provide an optical phase-change medium which can satisfy the need described above.
It is another and more specific object of the present invention to provide an optical phase-change medium which permits appropriate phase-difference tracking.
In order to achieve the above objects according to the present invention, a product for recording information by use of laser light includes a recording layer being able to change between an amorphous state and a crystalline state so as to record information, the recording layer having a thickness falling within a first range defined relative to a wavelength of the laser light, a protection layer having a thickness falling within a second range defined relative to the wavelength of the laser light, a heat releasing layer having a thickness falling within a third range defined relative to the wavelength of the laser light, and a transparent substrate supporting the layers. The first, second, and third ranges are selected such that a phase of the laser light reflected by the amorphous state and a phase of the laser light reflected by the crystalline state have a phase difference larger than a predetermined phase amount.
According to one aspect of the present invention, the phase difference is substantially larger than 10xc2x0. The larger the phase difference, the greater the output of phase-difference tracking becomes. As a matter of fact, the phase-difference-tracking output exceeds 0.3 V when the optical phase difference is lager than 10xc2x0. This means that an output level comparable to that of the DVD-ROM is obtained even when phase-difference tracking is employed. Namely, the optical phase-change recording medium as described above can be played on a DVD-ROM player.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.